1. Field of the Invention
The present invention relates to the field of high power laser systems for photo exposure and ablation, and more particularly to such systems utilizing a mask to define the pattern of the material to be exposed or removed by ablation.
2. Prior Art
The high power laser market has several applications for illumination systems which make use of the energy reflected from low duty cycle reticles (dark field masks). In the case of hole drilling with ultraviolet (UV) ablation, the holes patterned on a reticle are usually spaced far apart relative to their diameter, thereby defining only a very small percentage of the total area of the reticule. This type of reticle pattern "wastes" nearly all of the incident illumination energy because the energy reflected by the reticle is not captured and redirected through the openings in the reticle. Consequently, a relatively high power laser is normally needed to bath the entire reticule area in energy of sufficient intensity per unit area to remove the material in question at an adequate rate, yet the amount of energy actually productively used in material removal is no more than could be easily provided by a much lower power laser.
Mitsubishi Electric has disclosed a system concept which addresses the foregoing energy waste or loss (see Development of a High Efficiency Illuminator for Eximer Laser Machining, Masaaki Tanaka, et al., presented at Laser Thermal Machining Workshop, January, 1992). In that system, a flat plate reflector is placed in front of and parallel to the reticle. The laser provides a beam of the width of the reticle, but of only a small fraction of the height of the reticle. The parallel combination of the reticle and reflector are tilted forward slightly from being perpendicular to the optical axis of the laser, and the top of the reflector is positioned low enough to enable the laser beam to project just over the back of the reflector onto the upper part of the reticle. Assuming the reticle is mostly reflective, the beam will reflect from the reticle slightly downward to the reflector, reflecting therefrom back and forth in a downward continuous "Z" pattern between the reflector and reticle, illuminating the holes in the reticle where applicable, diminishing in intensity because of energy loss through the holes in the reticle, because of losses on each reflection and because of beam divergence. For a given reticle of a low percentage hole area, it is stated that this provides an energy utilization gain of 12 times over the conventional direct illumination systems.
The foregoing system concept has the advantage of being able to use lower power lasers and lasers operating at less than their maximum output to potentially obtain the same or higher material removal rates than normally obtainable with higher power lasers operating at or near their limits. However the achievement is not without its limitations and compromises. Because the beam from the laser is wide and thin, beam conditioning would be a problem, giving rise to non uniformities in the beam intensity in the horizontal direction. Also the distribution of energy in the vertical direction may be non uniform because of the inability to perfectly "stack" the rectangular re-reflections down the reticle. Because the system requires off-axis incidence in the initial beam and all re-reflections, one does not have a convenient way to form conical holes in the work piece of an arbitrary taper. In that regard, applications for such systems such as the manufacture of heads for ink jet printers may require not only small holes, but small holes with a predetermined conical shape. Finally, such a system would be very sensitive to the accuracy of the setup, including but not limited to the spacing and parallel alignment between the reticle and the mirror.
The following disclosure describes a simple illumination system which provides substantially uniform intensity illumination over an area or areas while increasing the usable intensity of a laser source by reusing much of the energy reflected from the reticle without special sensitivity to alignment of the parts of the system and without compromising the ability to produce tapered holes to specification.
The disclosure also describes a method and apparatus for controlling the time-intensity profile of the emission, for large area panel illumination and other aspects of the invention.